Morgoth
Melkor was not called "Morgoth" until he destroyed the Two Trees, murdered Finwë, the High King of the Noldor Elves, and stole the Silmarils in the First Age. The darker name was then bestowed by Fëanor, son of Finwë; and the Elves called him thereafter by that name alone. The name Morgoth is Sindarin (one of Tolkien's invented languages) and means "Dark Enemy", "Black Foe", or "Black Foe of the World". Bauglir is also Sindarin, meaning "Tyrant" or "Oppressor". Fëanor actually named him in Quenya(another of Tolkien's languages), Moriñgotto or Moriñgotho, and this was later translated into Sindarin as Morgoth. "Morgoth Bauglir" is thus an epithet. His name in Ainulindalë (the creation myth of Middle-earth and first section of The Silmarillion) is Melkor, which means "He Who Arises In Might" in Quenya. This too is an epithet since he, like all the Ainur, had another true name in Valarin (in the legendarium, the language of the Ainur before the beginning of Time), but this name was not recorded. The Sindarin equivalent of Melkor was Belegûr, but it was never used; instead a deliberately similar name Belegurth, meaning "Great Death", was employed. In earlier versions of Tolkien's legendarium the form of his name was Melko simply meaning "Mighty One". Like Sauron, he had a host of other titles: Lord of the Dark, the Dark Power of the North and Great Enemy. The Edain called him the Dark King and the Dark Power; theNúmenóreans corrupted by Sauron called him the Lord of All and the Giver of Freedom. He was called Master of Lies by Amlach. ☀Before the creation of Arda (The World), Melkor was the most powerful of the Ainur, the "angelic beings" created by Eru Ilúvatar (who is analogous to God in Tolkien's legendarium). Melkor, dissatisfied that Eru had abandoned the Void, had sought to emulate his creator and fill the Void with sentient beings. This, however, required the Flame Imperishable, the Secret Fire, which belonged to Eru alone; though Melkor searched for this, he could not find it. In what he hoped would be an alternative expression of his own originality and creativity, he contended with Eru in the Music of the Ainur, introducing what he perceived to be themes of his own. During the Great Music of the Ainur, Melkor attempted to alter the Music and introduced what he believed to be elements purely of his own design. As part of these efforts, he drew many weaker-willed Ainur to him, creating a counter to Eru’s main theme. Ironically, these attempts did not truly subvert the Music, but only elaborated Eru's original intentions: the Music of Eru took on depth and beauty precisely because of the strife and sadness Melkor's disharmonies (and measures to rectify them) introduced. Unlike his fellow Ainu Aulë, Melkor was too proud to admit that his creations were simply discoveries made possible entirely by Eru. Instead, Melkor aspired to the level of Eru, the true creator of all possibilities. Since the Great Music of the Ainur stood as template for all of history and all of material creation in the Middle-earth cycle (it was first sung before Time, and then the universewas made in its image), there was an aspect of everything in Middle-earth that came of Melkor’s malign influence; everything had been "corrupted". Tolkien elaborates on this in''Morgoth's Ring'', drawing an analogy between the One Ring, into which Sauron committed much of his power, and all of Arda – "Morgoth's Ring" – which contains and is corrupted by the residue of Melkor's power until the Remaking of the World.